This invention relates to the method and apparatus used to manufacture three-dimensional containers. More particularly, the invention relates to making three-dimensional trays from flat stock by cutting and pressing it into the desired form.
Trays of paper, coated paper, paper board, or laminated paperboard (hereinafter "paper") are commonly used as food containers; for example, in frozen food packaging and as fresh food containers in supermarkets and delicatessens. These inexpensive trays are made in large quantities by machines designed to transform rolls of flat paper stock into three-dimensional containers. High quality containers are not required by all customers, but very often the containers must meet strict uniformity standards in order to be acceptable. In such instances, each finished tray must be identical with the others. Achieving a highly uniform product in large quantities has proven to be very difficult.
Machines which have been developed to make three-dimensional containers (i.e., trays) from rolls of flat paper stock are quite complex. They feed the stock into the machine intermittently and movement of the paper stops while the tray is being made. After each tray (or set of trays depending on the machine design) has been made, it (or they) is ejected and the paper stock moved into position for the next tray to be formed.
Such machines normally cut blanks from a sheet of flat paper stock after first scoring the paper in a pattern which is appropriate for the type of tray to be formed. If the trays are shallow and do not require scoring, it is possible to slit the paper stock and then cut off pieces which are formed at the same time. The present invention relates to the manufacture of deeper trays which are scored before the trays are formed. The scoring permits the paper to be formed into the desired shape more easily. Scoring does not mean cutting through the paper stock, but refers to an operation in which a set of knives are pressed down into corresponding recesses in an opposed plate. The edges of the knives press into the paper to form indentations where the sides of the tray are to be made, particularly at the corners. The effect of the scoring will be familiar to anyone who has handled a tray made by pressing. After scoring, the paper stock is usually moved into another station where a scored blank is removed from the stock, either by punching or cutting. The separated blanks then are moved to a tray-forming station where they are formed over a mandrell, i.e., the male part of a die set having the shape of the tray.
Conventional paper tray-forming machines have a number of deficiencies. Some of the problems experienced with such machines should be mentioned so that the advantages achieved by the present invention can be appreciated. As just described, previous machines score the paper stock first, then cut and separate blanks from which the trays are formed and finally transfer the blanks to the tray-forming station. It is difficult to assure that each separated blank is moved to the forming station with the proper orientation. Blanks may turn, and it is common for blanks to reach the forming die improperly aligned. A tray produced from a misaligned blank may be rejected for defects, such as being misshapen or as having improper orientation of imprinted designs. Even if not rejected, the trays may be undesirably non-uniform. Non-uniform trays give a poor impression to the customer who receives them and do not work well in automated equipment. Also, non-uniform trays can stack poorly and take up more space in shipping containers, thereby increasing material and transportation costs.
Another problem associated with misalignment of paper blanks is related to the orientation of the fibers in the paper stock. It typically has the fibers principally aligned in one direction (the "machine direction"), as is the case with wood, and it is stronger in that direction. When the paper is formed over a die, force is applied to the paper while it is held by the edges of the blank. If the blank is misaligned, the paper may tear because of the variation in strength related to the fiber orientation. If the paper fibers in each blank are always aligned in the same direction when a tray is formed, then the effect of the variation in paper strength is always the same. Consequently, any variation in the final product resulting from the orientation of the fibers could be compensated for by changing the shape of the dies. This is not possible without consistent alignment of the blanks.
Still other problems relate to the moisture content of the paper. The paper stock may be preconditioned to provide a desired amount of moisture so that the paper can be formed over a die. If there is too little moisture, the paper may tear. If there is too much moisture, the paper will be distorted during the forming process. Also, just after the tray has been formed, relief of residual stresses from the forming process and drying of the paper can cause the tray to change shape. The tray will form the desired shape if it was properly oriented when it was formed. If not, then a permanently deformed tray may result.
It is common to heat the dies over which the paper stock is drawn during forming to assist removing the moisture and to help fix the shape of the tray. Excessive heating can cause softening of plastic coatings leading to sticking of the trays in the forming dies. Non-uniform thermal expansion of the dies may also cause problems in the manufacturing process. It will be appreciated that the clearances between the die parts are close and that heating one die may change such clearances.
Conventional tray-forming machines are difficult to adjust and maintain. Many of these problems are associated with the need to transfer a cut paper blank to the forming dies. It is typical that the blank is cut, and then transported by gravity to forming dies mounted at an angle to the horizontal. In that case, since the forming die set is mounted at an angle, it is very difficult to position and adjust in place because of its weight and size. Furthermore, mounting the die at an angle causes non-uniform wear on the moving parts. An important factor in properly forming a three-dimensional tray from a flat paper blank is maintaining the clearances between the die parts. They are more difficult to adjust and control when the forming die is mounted at an angle.
It was clear that a need existed for an improved tray-forming machine which would be easier to set up and adjust. If tray quality could be improved at the same time, making possible higher quality products and fewer rejects, a major advance in the tray-forming technology would have been made. The present inventors have achieved such results with a new tray-forming process and machine. It adopts a different principle of operation compared with that of the prior art machines. These benefits are made possible by simultaneously cutting and forming a tray from a strip of paper stock in a single die with a single stroke. This eliminates the problems relating to the cutting and transfer of a paper blank, as will be seen in the discussion which follows.